None? A lot of games are still single threaded. Maybe if you pick some games in particular we can give you better advice.
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tzenesSep 9 '10 at 2:15

2

Would it matter? Since your more likely being bound by your GPU (depending on the screen resolution) or your hard drive, so I would worry the least about your CPU influencing the performance
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Ivo FlipseSep 9 '10 at 8:02

1

@Ivo Is not 100% correct. First: HDDs only involve loading times not FPS (frame per second) performances. Second: CPU is not so marginal as people think (I tried recently on my skin). Using the same GPU as base you can have 10-30% FPS difference between various CPU (it depends also on the engine).
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DrakeSep 9 '10 at 21:30

@tzenes: A lot of games in the past maybe. None of the bigger games released in the last two years uses less than two cores. Even some games (Black Ops, if I remember correctly) fail to start on single core machines.
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DrFishDec 13 '10 at 14:28

Note that Source Engine may have it disabled by default. It's strangely in Settings' Video options tab as "Multicore rendering" (at least on CS:S/TF2).
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user2974Sep 9 '10 at 13:43

1

@Powerlord yes, correct. And indeed I found that apply this setting for a 3-core o 4-core CPU can boost your performance of from 5% to 15%. Not bad for a CPU-based setting modification :)
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DrakeSep 9 '10 at 21:24

If you look at it logically: Speedup in computers is done through parallelizing only (at least significantly). If you want to publish a game that takes advantage of current computers (and that's what most games want to do) you have to make your game ready for multithreading (see also this blog post).

Obviously parts like physics engine or KI can easily run each in a single process (if written like that!) which makes it easy to at least use some of the additional cores.